In recent years, magnetic disk units using a disk medium of 3.5 inches or smaller have become popular due to demand for compact disk units. As for disk media having a relatively small size, such as 3.5 inches or 2.5 inches, contradictory demands are placed on designers because there is high demand for large storage capacity and a simultaneous demand for a thin appearance. In order to raise storage capacity per disk, both bit recording density and track density must be increased at the same time. One way to increase bit recording density is to improve the head or medium, and modify modulation and demodulation circuits. To increase track density, head positioning accuracy must be increased.
With conventional head positioning, one side of a medium is regarded as a dedicated servo side. A servo head is placed over the servo side, and data heads on other media sides are positioned according to position information of the servo side read by the servo head. This is sometimes referred to as a servo side servo. However, this method can have problems because a mechanical mismatch may occur between the servo head and data head. This mismatch is generally referred to as an off-track. As the track density increases, this becomes a greater problem due to interference with adjoining tracks.
In order to resolve this off-track problem, it has become popular to make disk units having an embedded servo (sometimes called a sector servo) in which servo information is arranged at regular intervals in the form of sectors on the data side. When an embedded servo is used, a data reading head is used for positioning. Compared with the servo side servo, off-track can therefore be reduced significantly. Consequently, track density can be increased much more greatly than that permitted by the servo side servo.
Other disk units are available in which a MR head for reading and an inductive head for writing are mounted unitedly as a head capable of recording and reproducing information at a higher density. The MR head has a write gap and a separately formed read gap. Due to the orientation of the head positioned on an inner side of a disk and that of the head positioned on an outer side thereof, the centers of a write core and read core with respect to the track direction do not match with each other. An off-track therefore occurs between writing and reading. This is referred to as a yaw off-track caused by a rotary head actuator.
According to a track format (called a fixed-length block address format) based on the conventional embedded servo, the tracks have three areas: a servo area, an ID area, and a data area. These three areas are defined repeatedly in the track direction. In the writing operation, an ID area in which a cylinder address is pre-recorded is read and then a data area is written. If the center of the write core is aligned with the center of a track, the ID area is offset and read. A read margin is therefore reduced, and an error is likely to occur.
To avoid this, the ID area must be offset relative to the data area during formatting. Then writing can be performed. However, if the center of the read core is aligned with the data area during reading, since the ID area is read with it offset, the read margin is reduced, and an ID-area read error is likely to occur. When the center of the read core is aligned with the ID area, the write core in turn is off-track, which causes the read margin of the data area to diminish.
An ID-less format can be made by eliminating the ID area from the track format, as in Japanese Unexamined Patent Publication No. 5-174498. In a disk unit adopting the ID-less format, instead of eliminating the ID area, ID information such as a cylinder number, head number, or the like is recorded in the form of, for example, gray codes in a servo area. Thus, even if an off-track occurs, ID information reading can be achieved reliably, and head positioning for writing is guaranteed.
Using this ID-less format, it becomes unnecessary to align the center of the read core on a track with the center of the ID area and then perform reading. Writing and reading relative to the data area can be performed by aligning the centers of the write core and read core with the center of a track. However, in a conventional disk unit adopting the ID-less format, the recording density in the servo area is generally kept low in order to avoid a read error derived from an off-track. If track ID information such as a cylinder number, head number, or the like is placed in the form of gray codes or the like in-the servo area, a ratio of the area on the disk occupied by servo frames increases, and formatting efficiency falls.
In an effort to solve this problem, a method has been conceived in which track ID information or a logic block address (LBA) of data is placed in the data area that has undergone track formatting instead of placement of ID information in the servo area. Then the data area can be checked using a hard disk controller (HDC). However, although such a method can check the data during reading, it does not include a procedure of checking the data during writing, so there is a possibility that data may be written on an incorrect track.